For the first time, scientists have discovered that smell and taste are inseparable much earlier in the brain than we thought. New research shows that odors can actually be coded as tastes in the brain’s primary taste cortex, overturning decades of assumptions about how flavor is built.
Researchers from the Karolinska Institute in Sweden set out to test a long-standing assumption in sensory neuroscience – that taste and smell signals remain separate until they are integrated in higher brain regions such as the orbitofrontal cortex. Animal studies had hinted that this might not be the entire story, showing overlap in earlier processing areas, but until now there'd been no robust research on this process in human brains.
The team used functional MRI to record brain activity while exposing 20 healthy adults to the five basic tastes (sweet, sour, salty, bitter and umami) and to matching retronasal odors – smells delivered through the mouth to mimic how food releases aroma during chewing. They focused on the insula, the brain’s primary taste cortex, and used multivariate pattern analysis to compare the neural activity evoked by tastes and odors.
In everyday life, this is what we know as "flavor." When we eat or drink, taste on the tongue is combined with retronasal odors rising from the oral cavity into the nose. These odors are what make strawberries seem sweet and coffee seem bitter, even though the tongue itself can only detect five basic tastes.
What they found was that retronasal odors alone produced insula activity patterns that were not only distinct from unrelated smells, but also overlapped strongly with those generated by the corresponding tastes. Essentially, the smell of strawberries triggered insula responses that closely resembled those evoked by sweet taste, while a meaty odour produced patterns similar to umami taste. So our nose is an integral part of taste and flavor perception.
"We saw that the taste cortex reacts to taste-associated aromas as if they were real tastes,” explained lead author Putu Agus Khorisantono, from the Department of Clinical Neuroscience at Karolinska Institutet. “The finding provides a possible explanation for why we sometimes experience taste from smell alone, for example in flavored waters. This underscores how strongly odors and tastes work together to make food pleasurable, potentially inducing craving and encouraging overeating of certain foods.”
These findings challenge the long-held thought that taste and smell only combine in higher brain regions such as the orbitofrontal cortex, where signals are integrated with reward and emotion. Instead, these findings suggests that the brain treats certain smells as tastes right from the start, creating a shared “flavor code.” That helps explain why odors are often described using taste words – a wine’s “sweet notes” or cheese’s “savouriness,” for example – and why losing your sense of smell with a cold seems to switch taste off almost entirely.
“This shows that the brain does not process taste and smell separately, but rather creates a joint representation of the flavor experience in the taste cortex,” said Janina Seubert, senior researcher at the Department of Clinical Neuroscience. “This mechanism may be relevant for how our taste preferences and eating habits are formed and influenced.”
The study also uncovered a curious detail – in that the insula’s taste-related neural patterns weren’t static. They drifted over time in a way similar to what has been observed in rodents, suggesting that flavor maps in the brain are dynamic and flexible. This suggests that the brain doesn’t use a rigid flavor blueprint, but a more pliable one that can adapt with experience, adjusting to what you’ve recently eaten, what you expect to taste, or how memories shape perception.
While more research is needed, the discovery could have broad practical implications. Food scientists may use this knowledge to design healthier products that harness odor-induced taste coding, allowing flavors to "feel" sweeter or richer than they are, without adding sugar or fat. This understanding could also support therapies for people with impaired taste perception, by developing ways to boost flavor signaling through the nose.
The researchers will now look into whether this mechanism also applies to external smells – the kind we perceive by sniffing, also known as orthonasal odors.
“We want to find out whether the activation pattern in the brain’s taste cortex changes from salty to sweet when we walk from the cheese aisle to the pastries in the supermarket,” said Khorisantono. “If so, this could have a significant impact on the foods we choose to consume.”
Interestingly, in 2022, scientists found that the nose was responsible for the loss of taste and/or smell experienced by many people infected with COVID-19 – and not through congestion, like with a common cold. The virus muted the activity of olfactory receptors on nerve cells in the nose, making them unable to properly detect molecules associated with odors.
The study was published in the journal Nature Communications.
Source: Karolinska Institutet
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